Tracking Snowpack in a Warming World

NASA’s new airborne observatory gives unprecedented view of melting Sierra snow. New data may help transform how scientists think about snowpack and how a warming climate is changing it.

SAN FRANCISCO, CA., DEC. 9, 2013 — In May 1991, San Francisco came within two weeks of losing its primary water supply: snow melt in the Hetch Hetchy reservoir at the western edge of the Tuolumne River Basin that traverses the northern end of Yosemite National Park.

Reservoir operators had been flying largely blind. No reliable information was available to tell them how much snowpack blanketed the Tuolumne River Basin at the end of winter, what the average rate of melting was in early spring, and when peak snow melt crested rivers and streams in the basin.

View of the Tuolumne River Basin from the new Airborne Snow Observatory, mounted on a Twin Otter aircraft. Photo credit: NASA/JPL-Caltech.

Those days, however, may be over. This year, for the first time, a new observing system called the Airborne Snow Observatory mapped snow cover across the basin in unprecedented detail, measuring its depth and reflectivity every week from April 3 through June 8.

The result? In a year of continued drought and poor snowpack, Hetch Hetchy was full at the end of the end of the snow melt season, reservoir operators didn’t have to release any water that wasn’t needed, and they generated $3.9 million in hydropower.

“Airborne Snow Observatory data improves so many things, (and) reservoir operators are just desperate to have that kind of information,” Bruce McGurk, a hydrologist and former water and power planning manager at Hetch Hetchy, told an AGU audience in San Francisco on Monday.

The American West is sustained by mountain snowpack. Unlike the eastern seaboard and other parts of the country where heavy rain fills water tables, the West gets much of its water from snow. Mountain snowpack stores fresh water as natural reservoirs that melt throughout the spring and summer. In good years melting snowpack flows into streams and rivers steadily. But a warming climate has shortened winters in the West; the first day of spring in the Sierra is actually about two weeks earlier than it was 50 years ago, as measured by the timing of peak stream flows — which indicates the timing of peak snow melt. In a warming world, that means too much Sierra snowpack is often melting too soon and too fast — complicating how reservoir operators manage water supplies.

The new observatory, mounted on a Twin Otter aircraft, measures two aspects of snow cover that are critical to understanding how fast snowpack is melting: snow depth and snow reflectivity, also known as albedo. Instruments include a 3D Scanning LiDar (Light Detection and Ranging), which measures the depth of snowpack down to one-meter spatial resolution. The observatory also includes an imaging spectrometer, which measures snow albedo down to 2-meters resolution at 4,000 feet above ground level. Snow albedo is measured by subtracting reflected sunlight from incoming sunlight, which tells scientists how much sunlight the snow absorbs. Absorbed sunlight largely controls how fast snowpack melts.

The combined datasets tell scientists and water managers the total volume of water in snowpack in the watershed, and the absorption of sunlight that controls how fast it melts.

Since 1910, researchers have estimated the amount of water contained in snowpack in the Sierra Nevada by making manual ground snow surveys only at low to middle elevations. Meanwhile, the survey sites have covered only about 270 square feet in the Tuolumne Basin, so they’ve been far from comprehensive and often they’ve led to inaccurate forecasts.

The new airborne observations, by comparison, cover 460 square miles — with unprecedented accuracy in measurements.

The survey this year found that snowpack declined from early April through early June (65 days) from about 218,000 acre feet of water to a little more than 15,000 acre feet. Put another way, the amount fell from about 71 million gallons to just under 5 million gallons.

Jessica Lundquist, an associate professor of civil and environmental engineering at the University of Washington in Seattle, said the wealth of data coming from the Airborne Snow Observatory will transform how scientists think about snowpack and how climate change is changing it.

“The reason scientists are excited is we’ve been trying to figure out what’s going on there for years with almost no information. … Anytime you get new observations you start new science,” she said.

NASA’s Jet Propulsion Laboratory has so far invested $600,000 in the new observatory, while operating the survey flights will cost another $1.4 million for the three-year demonstration project that began this year, said Thomas H. Painter, a JPL scientist and principle investigator for the Airborne Snow Observatory. A third of the funding has been provided by the California Department of Water Resources, Painter said.

The Airborne Snow Observatory conducted a separate survey this year of the Uncompahgre watershed in Colorado’s Upper Colorado River Basin. In two years when the demonstration project is completed, the researchers say, they hope to expand the new surveys throughout the West.

Bruce Lieberman

Bruce Lieberman is a freelance writer covering science and environmental topics. He has more than 20 years experience in the news business. (E-mail: bruce@yaleclimateconnections.org, Twitter: @brucelieberman1)